The conventional navigator aboard a vehicle includes a first means for detecting running distance, and a second means for detecting the direction of motion of the vehicle. With the data obtained by these detecting means the coordinates of the position of the vehicle is calculated so as to indicate it on a display. To detect the running distance, detecting means detects a number of pulses generated in proportion to the rotation of the tires of the vehicle. The running distance is obtained by multiplying the number of pulses by a constant.
However, the diameters of the tires are not constant throughout the running distance due to changes in pneumatic pressure therein and/or in atmospheric temperatures, and the detection of the running distance is likely to contain errors caused by such changes in the diameters of the tires. In addition, errors are also likely to occur due to road conditions varying according to express ways, hilly roads, flat roads, and roads on rainy or snowy days. For these reasons detection of the running distance often leads to erroneous calculation of the position of the vehicle.